Your question can be best answered by obtaining a copy of the wall chart,

A CORRELATED HISTORY OF THE EARTH

From

Pan Terra, Inc.

PO Box 556

Hill City, South Dakota, USA

57745

I have two in my office, one on either side of the computer screen, and,

two more rolled up at home.

The largest, extinction producing impact craters have occurred

at 65 MYA , 252 MYA , 438 MYA , 625 MYA , ............. ( Group A)

There are smaller impact events within the ( 186.6 MY ) cyclicity.

There are roughly seven groups of impact times within each

186.6 MY cycle, with group A producing the largest impact craters,

and causing the greatest extinctions.

The approximate cycle in millions of years between groups is

A , 32, B, 20 , C , 20 , D , 34 , E , 14 , F , 17 , G , 49 , A = 186 MY

65 of 72 million years have passed, so the next group will be

a D group of impacts about 6 to 7 million years in the future.

Since most species live around 6 million years, and ours has

lived about 2 of the 6 million, leaving us 4 million left, we

will not be around anyway.

There will always be smaller space rocks hitting Earth at

various intervals, but they are seldom big enough to do

too much damage.

Well put! We have one of the Pan-Terra wall-charts here in the USGS Cascades Volcano Observatory.

I published an analysis of the Wabar impact site*, calculating the impact kinetic energy (12-20 ktons of TNT equivalent). Gene Shoemaker thought the probable kinetic energy at arrival in the upper atmosphere could have been up to 100 ktons of TNT equivalent). Arrival velocity is estimated at about 7-10 km/sec.

I also did some research as I was writing this article on other reported similar events, and came up with at least SIX "city buster" events in 127 years:

Wabar, 1863

Tunguska, 1908

Rio Curaca (Cuyuni), 1930

Rupununi, 1935

Sikhote-Alin, 1947

"Second Sun" event, SW Pacific, Oct 1, 1990 (Beatty, 1994).

We can also add Chelyabinsk, 2013.

This is a startling number of impact events - substantially higher than the asteroid impact flux estimates of Shoemaker (1983) and Bottke et al (2000). There may have been as many as 30-40 such events in the past century (see Beatty, 1994). Tunguska is estimated to have been ~80 meters in diameter, and Wabar was calculated to be only about 35 meters in diameter. It doesn't take much mass to destroy a city with just gravity-well velocities (11 km/sec or higher). Moreover, about 80% of the human population resides right now within 100 km of an ocean. A tsunami caused by an impact event in an ocean will have a substantially wider range of destruction.

==Jeff Wynn

US Geological Survey

* Wynn, J.C., 2002, Mapping an Iron-Meteorite Impact Site with a Magnetometer, and Implications for the Probability of a Catastrophic Impact on Earth: Journal of Environmental & Engineering Geophysics, vol. 7, no. 4, p. 143-150.

I've always thought that one of the interesting upshots of the meteoroid stream concept is that, if they exist, they could impact (pun intended) our crater counting models (which assume a constant rate in all size categories) over geologic time.

As for Chelyabinsk, I'm struck by its timeliness in helping to answer a question that has been in recent discussion: What is the lower size cutoff for making an object hazardous to human populations? With the Sandia Labs results raising the concern that the Tunguska bolide could have been as low as 30 meters, we can now say that 17-meter chondritic bodies are a concern. This value ramps up another notch the number with undefined orbits beyond Tunguska-class objects in Earth-crossing space. Iron bodies could presumably cause trouble at even smaller diameters. This assumes well-behaved objects following the power law size-frequency distribution. If we now find that meteoroid streams are contributing, the actual number may be difficult to estimate.

James Ashley

Arizona State University

If you look at the Moon and other smaller bodies, it is obvious that there have been

a lot of impacts happening over the last 4544.40 +/- 25.98 million years.

(My calculation )

It is also fairly obvious from the Surface of the Moon and the Surface of Mars that the

largest impactors have essentially been used up, and a whole series of the

largest sizes have been used up, but what is left is still large enough

occasionally to be any where from annoying and interesting, up to

cataclysmic.

I suspect that we get far fewer small ones than the planets without any, or with

little atmosphere, but that is because our atmosphere burns up and slows

down the smaller ones. The frequency of the larger ones would be similar

to Mercury, Mars, Venus, Moon, Titan, and all the others, except that the

earth can curve in a few more.

The frequency of impacts would be best calculated from the older and the

Younger surface areas of the other planets as we gain knowledge of

the ages of the surface areas of these other planets.

Our planet has this nasty liquid stuff (water) that keeps melting all the old evidence

away, and it gets covered with deep water over about 71% of the planet,

and it is really good at hiding the evidence of impacts.

Chyba et al, 1993, suggested that most stony or chondritic objects reach their structural limits in the atmosphere (at 10-25 km/sec speeds) in the 8-12 kilometer altitude range. At this point they fragment, and the surface exposed to atmospheric ablation increases dramatically. This sharply ramped-up ablation is what caused the reported flash that ignited much of the forest at Tunguska.

Gene Shoemaker observed that the cratering record on Earth "turns on" at about 3-5 kilometers in diameter (iron-nickel objects excepted). This implies that objects larger than about 150-250 meters in diameter will likely reach the Earth's surface, but that smaller objects will likely be broken up, like Tunguska. However, we know that the Wabar object was also broken into at least four large fragments (it had a highly oblique trajectory) within a kilometer of impact. This suggests that the Earth's atmosphere is even more effective than many had supposed. Wabar apparently touched the upper atmosphere relatively intact and held its structural integrity for most of its trajectory, to make such a tight crater distribution on the ground. Iron-nickel objects, however, constitute only about 2% of the NEO's.

"Orbiting rubble piles" (as Gene Shoemaker called them, based on the low density of 433 Eros), held together by their own microgravity, may be the dominant objects crossing Earth's orbit. Impact chains seen on Mars, the Moon, and with the Shoemaker-Levy 9 comet on Jupiter would also tend to support this. If so, the idea of a space tug, solar radiation pressure, or a nuclear device to deflect NEO's from an Earth-intersecting orbit is probably a non-starter. Whether solid or fragmented, the delivered kinetic energy will be the same.

==Jeff Wynn

US Geological Survey

All Earth geology is signed with the structures created by impacts.

The firs one is the collision of Mars and Earth. (Faulting is mirror like)

Meteorites have being, religiously, concept-ed as "exotic" events; but there are

frequent events., that Physics, cannot explain. Astronomy lacks serious knowledge about how rocks, and in general how really physical things, like rocks work & live.

Unfortunately Science is totally dominated by Stupid Religious constraints.

Good grief.

Dear Jeff and James, thanks for your good feedback. We all hope that others avoid this bench of scientific discussion.

The following story may be of some interest to the question

http://www.spacedaily.com/reports/Asteroid_named_as_possible_source_of_exploding_Russian_meteorite_999.html

Thank you, in fact I was the referee of that paper for MNRAS. You will find it in the arXiv repository: http://arxiv.org/abs/1307.7918

As stated in that paper, we first proposed in 2007 a rubble pile disruption during close approaches as also the origin of the 2002 NY40 meteoroid complex.

Thanks. I have just seen a paper in Nature

http://www.nature.com/news/russian-meteor-may-have-gangmates-in-tow-1.13498?WT.ec_id=NEWS-20130806

Happy International Asteroid Day, everyone!

📷: © NASA/Justin Cowart

Impact Geology is difficult under the monotonous Plate Shields geology. If you look to the big ones (Now expanded astroblemes) Take into consideration that about 250MMYA (Permian/Triassic) there was an enormous event which I discovered after the Yarabamba one (About 65MMYA) Go to Google Earth's -13.007900° -72.634585° and you will see an expanded astrobleme with more than 300 Km with all the anular & radial structures (half of it is covered by recent volcanics) and one thing more...meteorites tend to fall to were big impactswere (Mascon effect) One day I will make public my hypothesis of Onion Earth vs Plate Tectonics...

Fortunately the really big impactors a rare occurring at around 186.6 million years apart. 66.0, 252.4, 439 Ma etc.

However, small impactors are numerous, but fortunately, the chances of one zapping a city are fairly small. 71 % will hit oceans, 29% will hit land, and of that 29%, only about 1 % will hit a city.